The present invention relates to optical instruments such as telescopic sights, riflescopes, surveying telescopes, and the like, for the accurate alignment thereof with objects of interest. More particularly, the present invention relates to the reticles employed by these classes of instruments.
It is well-known in the art of telescopic sights, such as riflescopes and surveying telescopes, which comprise an objective lens, or lenses, and an ocular lens, or lenses, and image-erecting optics, that images of objects at different distances from an operator, being viewed by means of the optical instrument, focus at different points along the internal optical axis of the instrument. For instance, the image of a near object being viewed by an operator through the optics of such an instrument is focused at a point somewhat more rearward (closer to the ocular end) on the optical axis than is the image of a more distant object which will focus farther away from the ocular end of the optical instrument. When the optical instrument is equipped with a fixed alignment reticle, as in the case of a riflescope, this shift in focus introduces parallax error between the image of the object and that of the reticle. This means that the axially-fixed alignment reticle will not be on the same plane as the image of the object formed by the objective lens system. In this case, if the operator's eye is not perfectly aligned with the optical axis of the instrument, the images of the object and the reticle will not coincide. The result is misalignment of the object image with the image of the reticle which degrades the accuracy of optical instruments that are required to provide precise alignment of the object image with that of the reticle image.
In riflescopes, parallax error is generated by the instrument being focused at a distance different from that of the target being viewed thereby in combination with the lateral displacement of the operator's eye from the optical axis. The importance of centering the eye on the optical axis of the riflescope is extremely important and is easily demonstrated. Typically, riflescopes have an eye relief of three inches to allow adequate space between the rear of the scope and the shooter's eye so that, when the rifle is discharged, the apparatus does not travel far enough back under recoil to strike the shooter's face. Considering a reasonably close target distance of one hundred yards, a one-degree deviation of the shooter's eye off-axis (a mere 0.087″ [2.2 mm] to one side) will result in a parallax error at the target of sixty-three inches. In other words, the aim is off by a distance of over five feet even though the shooter may have, otherwise, perfectly aligned the image of the crosshairs with the image of the target. Targets at greater distances will generate increasingly greater parallax error. Thus, in the example given, if the distance is increased to 200 yards, the error would double to over ten feet.
Thus, when these two negative factors, parallax and displacement of the operator's eye from the optical axis, are combined, as they often are, the resulting error can be overwhelmingly great. The prior art has failed to provide visual feedback to the operator of the presence of these deleterious conditions. Until the present invention, there has been absolutely no way for the operator of instruments of these classes to be alerted to these negative conditions. The present invention remedies these defects of the prior art by providing visual indicators that serve to neutralize these negative factors that work against the accuracy of these instruments.
Because of the potentially significant shift in the point of focus of images of objects of interest due to variations in the distance from the operator of objects of interest, some means must be provided to compensate for the discrepancies of axial alignment to eliminate the deleterious effects of parallax error which render optical alignment inaccurate for purposes where precise alignment is critical, such as the aiming of handheld hunting rifles, tactical weapons, and surveying telescopes.
The prior art, such as U.S. Pat. No. 4,072,396, issued Feb. 7, 1978, to Cecil Jack Ross, has addressed this problem by providing mechanical means for the adjustment of the position of the objective lens system along the optical axis so as to move the image formed thereby to coincide with that of the alignment reticle. A severe limitation of the Ross device is that it requires coordination of the operator's hand (to turn the objective lens barrel) and eye (to observe the ‘approximate’ range-focus setting). Another major limitation of this device is it requires the user to estimate the distance to the target.
A similar approach is taught by U.S. Pat. No. 3,516,736, issued Jun. 23, 1970, to William R. Weaver. The Weaver structure teaches a focusing objective for telescope sights for firearms employing a slidable lens cell and a threaded drive collar for urging the lens cell in one direction in opposition to a biasing means urging the lens cell in the opposite direction for the purpose of focusing a target image on the same plane occupied by a reticle within the telescopic sight.
Another example of the prior art that seeks to overcome the problem of parallax in the alignment of optical images is taught by U.S. Pat. No. 3,336,831, issued Aug. 22, 1967, to J. Unertl, Jr. As with Ross and Weaver, Unertl utilizes a moving objective lens assembly. But, in the case of Unertl, the objective lens components are moved by a cam and pin arrangement which, according to the patent, provides greater control of the movement of the objective components. But, as in the structures of Ross and Weaver, Unertl's device requires the coordination of hand, to perform a physical act (adjustment of the objective lens barrel) and eye of the operator (to observe the ‘approximate’ range-focus setting). And, as with Ross, a limitation of the Unertl device is it depends upon the user to provide an accurate estimate of target distance.
Another, and severe, drawback to the devices of the prior art such as those taught by Ross, Weaver and Unertl is that the excessive time required to estimate the target's distance and manually set the range focus before acquiring a sight picture of the target can easily mean that the target of interest may move out of sight or become aware of the user's presence due to the additional time and movements required by the act of range focusing.
Perhaps the greatest drawback to the prior art as exemplified by the above references is their potential for leakage at the interface between the adjustable objective lens and the housing of the instrument. Even the smallest amount of moisture is disastrous to optical telescopes because, once inside, it vaporizes, fogging the inside lens surfaces, totally obscuring visibility through the instrument. For this reason, modern manufacturing practice in these classes of products include a final step of vacuum purging to remove all ambient air, replacing it with pure nitrogen gas which is totally inert, and, then, hermetically sealing the instrument. Such sealing is difficult to maintain in devices that utilize range-focusing objectives.
A different approach to the aiming of a firearm is taught in U.S. Pat. No. 5,208,989, issued May 11, 1993, to Ronald J. Sanders. The Sanders device is directed to the aiming of a slug barrel shotgun which is a short range firearm requiring no compensation for parallax due to long range firing requirements of hunting and tactical rifled firearms and surveying instruments as contemplated by the present invention. Sanders teaches a sighting device with two plano-plano reticles, each bearing an illuminated aiming reference. There is no magnification in the optics of the Sanders device which is intended for use with firearms of the pistol class and non-firearms that require extended eye relief mounting. The Sanders structures are more akin to ‘notch and bead’ aiming arrangements than to optical instruments such as riflescopes. Thus, in applications addressed by Sanders, there is no parallax problem as in riflescopes of the magnified variety as addressed by the teachings of the present invention.
U.S. Pat. No. 6,453,595, issued Sep. 24, 2002, to Dennis J. Sammut, which is a continuation-in-part of application Ser. No. 09/129,729, filed on Aug. 5, 1998, now U.S. Pat. No. 6,032,374, which is a continuation-in-part of application Ser. No. 08/986,458, filed on Dec. 8, 1997, now U.S. Pat. No. 5,920,995, discloses a reticle for riflescopes having a single so-called “ghost ring” axially centered and encircling a variety of aiming indicia, including crosshairs. The purpose of the “ghost ring” is to aid in the centering of the user's eye on the optical axis. However, how this single “ghost ring” aides the user is not entirely clear. At column 12, lines 51-53, it is stated that “By insuring that the ghost ring 41 is centered within the field of view of the scope, the reticle will likewise be centered.” However, because the overall field of view through a scope is not readily discernible by an operator it is a very unreliable reference with which to judge centering of the “ghost ring”. The reason for this is the fact that riflescopes are, as previously mentioned, designed to provide approximately three inches of eye relief (the distance from the end of the scope to the user's eye) to prevent injury from kick-back when the firearm is discharged. This distance presents the user with a sight picture that is slightly less than the overall field of view of the scope. Thus, the only thing visible to the user, other than the sight picture, is the outer ring of the ocular housing which, at three inches, appears extremely blurry and indistinct.